The inductive effect
is an experimentally observable effect of the transmission of charge
through a chain of atoms
in a molecule
by electrostatic induction
(IUPAC definition). The net polar effect
exerted by a substituent
is a combination of this inductive effect and the mesomeric effect
The electron cloud in a σ-bond between two unlike atoms is not uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge(δ-) and the other atom has a slight positive charge(δ+).
If the electronegative atom is then joined to a chain of atoms, usually carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the -I effect.
Some groups, such as the alkyl group are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing. This is electron releasing character is indicated by the +I effect.
As the induced change in polarity is less than the original polarity, the inductive effect rapidly dies out, and is significant only over a short distance. The inductive effect is permanent but feeble, as it involves the shift of strongly held σ-bond electrons, and other stronger factors may overshadow this effect.
The inductive effect may be caused by some molecules also. Relative inductive effects have been experimentally measured with reference to hydrogen.
Inductive effects can be measured through the Hammett equation.
- Aliphatic carboxylic acids. The strength of a carboxylic acid depends on the extent of its ionization-the more the ionization, the stronger is the acid. The strength of an acid is denoted by the numerical value of pKa. In aliphatic acids, the electron releasing inductive effect of the methyl group, increases the electron density on oxygen and thus hinders the breaking of the O-H bond, which consequently reduces the ionization. Greater ionization in formic acid when compared to acetic acid makes formic acid(pKa=3.75) stronger than acetic acid(pKa=4.76). Monochloroacetic acid(pKa=2.82) is stronger than formic acid since the electron-withdrawing effect of chlorine promotes ionization.
- Aromatic carboxylic acids. In benzoic acid, the carbon atoms which are present in the ring are sp2 hybridised.As a result, benzoic acid(pKa=4.20) is a stronger acid than cyclohexane carboxylic acid(pKa=4.87). Also, electron-withdrawing groups substituted at the ortho and para positions, enhance the acid strength.
- Dioic acids. Since the carboxyl group is itself an electron-withdrawing group, the dioic acids are, in general, stronger than their monocarboxyl analogues.
- In the so-called Baker-Nathan effect the observed order in electron-releasing alkyl substituents is apparently reversed.